Phineas and Ferb do something big everyday. Often those are massive projects that would require a seroius/millionare amount of money.
Is there any explanation in or out of universe about the fundings Phineas and Ferb use to build their projects?
As of my Knowledge, this question makes little sense and is hypothetical, because there exists no Universe where the summer holidays never end!!
Although after surfing on the net, I found a descriptive answer (though not fully satisfying).
Money for inventions , its safe to assume that they get some sort of Pocket Money and also they don’t waste that money on anything , cause why would they? (I know that pocket money isn't enough)
Also , also the people who supply the materials are cool enough ,when they say this- Aren't You a Little Young” for all the time, but on the other hand, Phineas always says that , “Yes , I am ” : )
The best case scenario is when they have their pocket money that gets them enough for their first project (invention) , consider when they built the roller coaster or the Elevator to the moon , people pay for that , and they charge for it (please , let me in that , I will pay you whatever you need , that’s possible) , right ( It may be correct) . I know the kids don’t pay that much but we will consider it. "
Also, we literally don't know that Phineas and Ferb know that if Perry is an agent. What if they do know, and make a deal with Perry with the bonus he always gets after defeating Doofenshmirtz.
In some cases, they may also recycle the broken parts of Dr. Heinz Doofenshmirtz Machines.
Well, I would only like to conclude that there could be many possible theories. Since, it is only an animation, we can't come to a final conclusion. It still will remain a mystery!
Probably they sold all their inventions as prototypes/finished products. We never see them permanently possessing any of their highly expensive and extravagant inventions. Also, they are not always destroyed. So it is safe to assume that someone anonymous pays them for all their inventions. Someone they themselves didn't know about. It might be Doofenshmirtz, as he is always related to their inventions, one way or the other; but it could also be OWCA (or both). We never saw the rollercoaster tracks destroyed, did we?
Their inventions are always successful. They even get permissions for their construction (a minor cannot get permissions)! So there must be a group funding them anonymously (and being responsible for them) and profiting out of their inventions. Lot of people could use the profits. For instance, someone has to raise funds for OWCA, and for Doofenshmirtz's evil inventions. They don't have a good business model and keep losing money all the time. It could all be funded by Phineas and Ferb's inventions. After all, all their activities are closely connected. If either of these three parties, Phineas and Ferb or Doofenshmirtz or OWCA stop their activities, the cycle always breaks. Why could that be? Because they depend on the kids inventions to fund their "good guy - evil guy" game. And Phineas and Ferb are always able to pull off their invention.
Desclaimer: None of this is official, this is just my fan theory.